I have read in the documentation about automatic /lightweight migration for Core Data models - but am having problems in the reality of implementing it.

As I understand it the application should notice that the model it has and the model that exists on a device already are not the same. If you have only added attributes or relationships and similar simple changes then the model should be upgraded automatically.

Select the Design Menu at the top - then Data Model - then choose Add Model Version

Your xcdatamodel file will then get moved into a new directory with the same name as your xcdatamodel file but with the extension xcdatamodeld - there will be a second file in this directory with a 2 in the name. Select the new file and then Design->Data Model->Set Current Version (in Xcode 4 you do this)

If you have already made the changes that have caused your project to be incompatible - take these changes out of the original xcdatamodel file. If you have yet to make the changes - then just edit the 2.xcdatamodel file (the one you just made current version).

Now when you install this version onto a device that has the old model - it will automatically upgrade that model to the new model.

This seems great and as simple as I wanted - but I think you need to be careful during development as you change a model - otherwise you will have to create a new version for each change.

I think what I will do is that I will keep all of the changed files and then once I get ready to deploy my update I'll delete all the in-between files and just deploy with the oldest and latest models.

UPDATE (15/07/2011):

Thanks to @rockstarberlin for pointing out there is updated documentation at apple:

I'd suggesting reading the Core Data Migration and Versioning Guide, available in the Xcode doc viewer and on developer.apple.com.
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HunterJun 19 '09 at 15:10

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I didn't find the answer in there laid out simply - it took a while to find out how to do this.
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GrouchalJun 19 '09 at 16:03

NSInferMappingModelAutomaticallyOption works well, but only for simple mappings, like changing the name of an attribute. For anything more complicated (relationships, removing or adding entities) you will need to add a mapping model. If Xcode complains that NSInferMappingModelAutomaticallyOption is undeclared, add #import <CoreData/CoreData.h> to your app delegate header file.
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Elise van LooijSep 29 '09 at 14:13

I added some entities to my model and got the infamous "The model used to open the store is incompatible with the one used to create the store" error. Your solution fixed it! Thank you very much!
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Karsten SilzMar 15 '10 at 23:31

1

See Santthosh's answer below if you are getting "can't merge models with two different entities named" error after following these instructions.
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benvolioTNov 17 '10 at 3:26

This was incredibly helpful. The Apple documentation was -- as usual -- woefully incomplete. I recommend doing a clean build, as I ran into an error "Can't merge models with two different entities xxx" when I first ran after making these changes. The clean build fixed it up.

Grouchal's answer is perfect...but if you are still having the "Can't merge models with two different entities xxx" even after cleaning up the build several times...Your might have issues with how the managedObjectModel is being loaded...take at look at this one...which helped me fix it..

Also, if you stumbled upon this post, like I did, after getting the "The model used to open the store is incompatible with the one used to create the store" error and you are just debugging using the simulator and wanting to completely replace the old model installed, you can just Reset the Simulator app or deleting your app from the simulator would probably work as well.

It didn't occur to me to try this until reading the posts here, at which point I realized that I had installed the app in the simulator and then subsequently changed the model, causing the aforementioned run-time error.

To follow up on Santthosh's answer, figured I'd post the code snippet right here instead. You need to create your managedObjectModel with initWithContentsOfURL: instead of mergedModelFromBundles: otherwise you'll get error:

Can't merge models with two different
entities XXX and XXX

If your Model file is named "Model", here's how you create the managedObjectModel:

I've had this issue for years, and I tried all of these answers to no avail. Today I finally figured out what I was doing wrong. Very simple problem, but I overlooked it. When creating a newer version of the data model, if you are ADDING columns make sure to mark them as OPTIONAL. If you do not the simple migration will not work because the new column values will not be filled in.

As soon as I made sure my new columns has "optional" checked, I tried the migration again and it worked.

I stumbled onto this post because of a different problems, but the error was "The model configuration used to open the store is incompatible with the one that was used to create the store."

Here was my problem and the solution to it. In my model, I was using configurations. I had some of the entities being stored in one file and the others in a second file. (I have some defaults that might periodically need to get downloaded, and it would be an incredible pain to merge them into the whole). Anyhow, I made a new entity. The program seemed to run fine, but whenever I'd quit, I got the above error.

The solution there was to look at my configurations, realize that I had an entity that wasn't currently in any of the configurations, and add it to one. Runs like a dream.

This won't fix the OP's problem. But maybe some frustrated person who lands here via google will be in the boat I was in :)